[TowerTalk] Hi-Tech Station Cost (>QUADS)

[K3BU@aol.com]

In a message dated 5/30/2000 20:39:51 Eastern Daylight Time, 
w8ji@contesting.com writes:

> 
>  Q has several definitions, and so it also has several ways to define 
>  the value of Q, Yuri.
>  
>  For example:
>  
>  Q can be the bandwidth of a system at a certain SWR value. We 
>  sometimes see the VSWR bandwidth of an antenna measured and 
>  that value used to assign a "Q" value.
>  
>  Q can be the ratio of reactance to resistance. In this case Q 
>  expresses either the ratio of resistance (which can be either power 
>  wasted as dissipation or power needed by a load to do something 
>  useful) to reactance.
>  
>  Q can be an unloaded value, or a loaded value where the source 
>  and/or load impedances affect the Q. Tank circuits are defined this 
>  way, when we say a linear amplifier's tank has a "Q of 12" (or 
>  whatever value).
>  
>  So what way are you using Q, and what does Q have to do with 
>  gain?? 
>  73, Tom W8JI

Sorry about the delay, family affairs and contests kept me busy.
   I was talking about Q of antenna elements - 1. full wave quad loop, which 
could be considered to be "stretched out" or variation of folded dipole, and 
2. half wave single wire (tubing) dipole element. 
   Let me quote from the Bill Orr, W6SAI book: All about Cubical Quad 
Antennas, 2nd edition, page 13:
   "...  An interesting side effect is apparent when the multi-wire dipole is 
compared to the single wire version. The bandwidth of the single dipole is 
quite narrow compared to the bandwidth of the folded dipole. That is, the 
circuit "Q" of the folded dipole is low in comparison to the "Q" of the 
single dipole. The folded dipole may be thought of as a broad band, low "Q" 
system as compared to the characteristics of the normal dipole. This means 
that the impedance at the tips of the folded dipole is much lower than that 
value noted at the tips of the single wire dipole (figure 2). (Plots show 
that folded dipole has lower resonant frequency and lower voltages at the 
ends for the same physical length of the elements.)

This fact is an important consideration when the antenna is used in 
conjunction with a high powered transmitter. Generally speaking, for a given 
amount of impressed power, higher values of r-f voltage exist at the high 
impedance points in any antenna that at the low impedance points, and the 
amount of r-f voltage at any point is proportional to the antenna impedance 
at that point. The voltage distribution curves typical of these two types of 
antennas are shown in figure 2B. The folded dipole has a measurably lower 
value of r-f voltage at the extremities, and is less susceptible to corona 
discharge and other undesirable high voltage phenomena."

   Q of the element, just like in tuned circuits, contributes to the way they 
couple to other resonant circuits (antenna elements) in the array. Higher Q 
element (Yagi) will be more "sensitive" as a parasitic element, while lower Q 
element (Quad loop, folded dipole) will be less "sensitive" but more 
broadband.

   This has shown when I did my experiments with Razors, when I tried to go 
with more than one quad parasitic directors, the gain was less when I used 
dipole elements. I tried various combinations, but I ended up with only one 
quad parasitic element on the front and behind the driven element or log 
cell. If I replaced that first director with dipole, it exhibited less gain 
than quad element in its place.

   I hope this explains "which" or how Q affects the antenna design. It 
appears that Q is important to consider in parasitic arrays, while in driven 
arrays it might not by as important and could actually help with broadbanding 
the antenna.
 
   K8CFU also found experimentally that folded radiator in vertical antenna 
exhibited "surprisingly" small gain over "plain" vertical made out of tubing.

Yuri, K3BU, VE3BMV

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